Variability and extremes of poleward breaking Rossby waves over the North Atlantic-European region

Variability and extremes of poleward breaking Rossby waves over the North Atlantic-European region (VARNAER)

Phase 1: Dieter H. W. Peters and Andrea Schneidereit

Leibniz Institut für Atmosphärenphysik an der Universität Rostock

Ostseebad Kühlungsborn, Mecklenburg

Summary

In the extratropics Rossby waves play an important role in determining the general circulation, especially in the upper troposphere / lower stratosphere region. It is known that events of poleward breaking Rossby waves are often observed over the North Atlantic-European region in wintertime. In this project we investigate the influence of the observed background flow in the upper troposphere on poleward Rossby wave breaking events and the link to severe weather. We will study the intra-seasonal and interannual variability of events of poleward breaking Rossby waves over the North Atlantic-European region and diagnose the extreme cases of the wave breaking events for each season, based on ECMWF Reanalysis (ERA-INTERIM) and analyses. The predictability of such events is investigated by using different forecasts data sets. Furthermore, the expected influence of the different zonally varying background flows on poleward Rossby wave breaking will be examined in simplified global ECHAM5 model simulations in order to improve the understanding of this process. A further key investigation is the diagnosis and mesoscale modelling (WRF) for implications of poleward Rossby wave breaking events on the change of the Grosswetterlage (large scale flow) in the lower troposphere, in order to examine different acting mechanisms, and to understand the sub-seasonal differences of the influence and of the impact on severe weather over Northern Europe.

In the Phase 2 we will focus on the investigation of the sub‐seasonal variability of poleward Rossby wave breaking events and their extremes. Especially, the sub‐seasonal prediction is a very important issue for future research and a part of THORPEX objectives to focus on not only in Europe. Sub‐seasonal prediction (or intra‐seasonal prediction with typical timescales between 10 and 90 days) has got a much higher priority for THORPEX since the original proposal, especially due to the joint focus with WCRP on seamless prediction. Thus the new direction in our project shows that PANDOWAE is developing in response to changed THORPEX priorities. The seasonal cycles of RWB events, their inter‐annual variability and extremes have been examined in Phase 1. As a logical extension three new and important questions are taken into account:

(i) How does the sub‐seasonal variability of poleward RWB events look like? Is there a significant sub‐seasonal impact on severe weather variability?

(ii) Which are the main stratospheric or tropospheric influences on the subseasonal variability of RWB events?

(iii) Which processes are responsible for an enhanced sub‐seasonal prediction?

The focus in Phase 2 will be laid on the investigation of sub‐seasonal variability of the background flow and on those atmospheric processes which have an influence on this

variability because that determines the behavior of RWB events and their induced impacts. The link to RWB events has been established in Phase 1 but it should be

examined also for the sub‐seasonal variability. The influence of long‐term variability on the sub‐seasonal variability should also be considered. From former studies we know

that two main influences could play an important role in determining the zonally asymmetric background flow and induced RWB events and their variability:

(i) Stratospheric‐tropospheric coupling: After an enhanced tropospheric forcing of planetary waves, these waves could propagate into the stratosphere. Due to this amplified propagation especially in winter time, a shift or splitting of the polar night vortex often occurred, inducing minor or major sudden stratospheric warming (SSW) events. The evolution of these SSW events is a typical example of a sub‐seasonal temporal variation, which influences the quasi‐stationary planetary wave field down to the troposphere over more than 40 days which may have a strong influence on upper tropospheric RWB events due to changing the background field significantly. An influence of long‐term variability like QBO on SSW is also known.

(ii) Tropospheric variability: The projection of the divergence of transient baroclinic waves on sub‐seasonal time‐scales is known to change the background flow due to changing the eddy‐forcing of planetary waves. Furthermore, it is known that the intra‐seasonal outgoing longwave radiation (OLR) variability has a significant projection on extra‐tropical upper troposphere flow structure. Both processes should have an influence on RWB events. It is expected that this sub‐seasonal behavior is influenced by atmospheric oscillations like NAO (which themselves could be the result of differential RWB processes) or ENSO due to the interaction with subtropical processes or underlying oceans.

All in all the proposed research focused on the main atmospheric processes which determine the structure of the sub‐seasonal variations of the zonally asymmetric background flow in the upper troposphere where the triggering of Rossby waves occurs, Rossby wave trains are observed, and the Rossby waves break.

Motivation:

A model pilot experiment was performed to investigate the role of stratospheric processes on RWB events. A SGCM experiment was run with a change of the location of the polar vortex due to a wave-1 disturbance in the stratosphere. In the zonally asymmetric case (left panel) the results show enhanced RWB-P2 events to occur more locally over the North Atlantic. We found that a shift of the polar vortex influences the position of RWB events sensitively (A. Gabriel personal communication):